Planting Guides
When to Plant Onions in Indianapolis: Complete Guide + Best Varieties for Zone 5b
Indianapolis, Indiana
USDA Zone 5b
Last Frost: May 5
Last updated: October 30, 2025
Plant onions in Indianapolis from March 15-April 15 for Zone 5b success. Long-day varieties like Walla Walla and Red Wing thrive in Indiana's 158-day growing season.
— Image © PlantReference.org 2026
Quick Answer
Plant onions in Indianapolis March 15-April 15 using long-day varieties. Harvest occurs early October after Zone 5b's 158-day season.
TL;DR
Plant onions in Indianapolis from March 15 to April 15 for Zone 5b success. Choose long-day varieties like Allium cepa 'Walla Walla' and Allium cepa 'Red Wing' that need 14+ hour days to bulb properly. Start garlic Allium sativum 'Music' October 1-15, and harvest onions in early October after 158-day growing season.
Frequently Asked Questions
When exactly should I plant onions in Indianapolis for the best harvest?
Plant onions in Indianapolis between March 15 and April 15 for optimal results in Zone 5b. This timing provides 4-6 weeks before the May 4 last frost date, allowing plants to establish strong root systems. Early planting maximizes the vegetative growth phase before summer's long days trigger bulb formation. Transplants work best planted March 15-April 1, while onion sets can go in slightly earlier from March 1-30 since they tolerate cold better.
What onion varieties grow best in Indianapolis's Zone 5b climate?
Long-day varieties are essential for Indianapolis success due to our 39.7°N latitude. Choose varieties requiring 14-16 hours of daylight to trigger bulbing, such as Allium cepa 'Walla Walla' for fresh eating, Allium cepa 'Red Wing' for long-term storage, or Allium cepa 'Yellow Sweet Spanish' for large bulbs. Avoid short-day or intermediate-day varieties - they won't bulb properly in Indianapolis because our summer daylight never reaches their trigger requirements.
Can I plant onions in fall in Indianapolis like I've seen recommended for southern areas?
No, fall onion planting doesn't work in Indianapolis for bulb production. Our Zone 5b winters are too harsh for overwintering onion plants, and long-day varieties need spring planting anyway. However, plant garlic in fall - Allium sativum 'Music' cloves planted October 1-15 will establish roots before winter, undergo necessary cold treatment, and produce large bulbs for July harvest. Focus on spring onion planting and fall garlic planting in Indianapolis.
Why did my onions produce huge green tops but tiny bulbs last year?
This indicates you planted short-day or intermediate-day varieties instead of long-day types required for Indianapolis. When day-length requirements exceed available photoperiod, plants remain vegetative indefinitely without bulbing. Indianapolis needs varieties triggered by 14+ hour days, which we reach in June-July. Stick to verified long-day varieties like those sold specifically for northern climates to ensure proper bulb development.
How do I know when my Indianapolis onions are ready to harvest?
Begin harvest when 50-75% of tops fall over naturally, typically late August through early October depending on planting date. Don't force tops over by bending them - this stops carbohydrate flow to bulbs. Look for yellowing from bottom up, papery brown outer skins, and firm bulbs when squeezed gently. Indianapolis's dry September weather provides ideal curing conditions - lay harvested onions in single layers in ventilated, covered areas for 2-4 weeks until necks feel tight and wiry.
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Priya Sharma
Priya is a soil scientist at an agricultural extension office who gardens on a quarter-acre suburban lot in Indianapolis. Her professional work focuses on soil health and nutrient management, which gives her a perspective most gardening writers don't have—she thinks about what's happening underground before worrying about what's happening above it. Priya maintains a large pollinator garden, grows herbs and vegetables, and is slowly converting her conventional lawn to a mix of native grasses and low-growing groundcovers. She writes about soil health, composting, fertilizer use, and building the kind of foundation that makes plants thrive without constant intervention.
